1. Field of the Invention
The present invention relates to a semiconductor laser device and method of fabricating the same, and, in particular to a device in which a width of a current pass region is controlled in the device, thereby stabilizing a single transverse mode.
2. Description of the Related Art
Four-element systems, as such as InGaAsP, AlGaInAs, etc., are commonly in use as InP-based semiconductor compounds for optical communication semiconductor lasers, and recently, AlGaInAs based materials have become spotlighted to a great extent, as a result of rising demand for greater high-temperature properties and high-speed modulation properties of optical communication devices. AlGaInAs based materials have good temperature properties due to large band offset in a conduction band and properties advantageous in high modulation due to small band offset in a valence band, as compared to InGaAsP based materials. However, because AlGaInAs based materials contain aluminum, oxidation film is naturally formed when the AlGaInAs based materials are exposed to the air; therefore, re-growth is not easy, and thus it is difficult to implement a device. Accordingly, when devices are fabricated using AlGaInAs based materials, an RWG (Ridge Wave Guide), a BR (Buried Ridge) or the like is usually used. Such an RWG or BR forms a ridge, thereby performing current injection and optical guiding, which has been widely used until now as a technique for fabricating laser diodes without exposing an active region.
FIG. 1 is a cross-sectional view schematically showing a construction of an RWG type laser device 10. Referring to FIG. 1, the RWG type laser device 10 is a type having a projected ridge 16, which is formed principally in a reversed trapezoid form, and is formed from a clad layer 15. The layers underlying the clad layer 15 are, in order, an etch stop layer 14, a space layer 13, an active layer 12 and a substrate 11.
However, such a conventional RWG laser device 10 is fragile to physical impact, because the ridge 16 is projected. The ridge 16, is therefore likely to be fractured during the fabrication process. Furthermore, it is very difficult to form an ohmic contact layer on a mesa whose size is a few micrometers. In addition, such a projected ridge may be the cause of increasing chip resistance.
It is also necessary to reduce the thickness of the space layer 13 to mitigate current spreading in an area adjacent to the active layer 12. In so doing, however, optical guiding becomes too intensive in trenches 17, thereby deteriorating a mode characteristic and causing a kink.
Implementing an optical communication device of high quality is problematic, due to limits on mesa width and space layer thickness reduction for the conventional RWG type laser device.
FIG. 2 is a cross-sectional view schematically showing a construction of a conventional BR type laser device 20 comprising a substrate 21, an active layer 22, a space layer 23, an etch stop layer 24, a current blocking layer (CBL) 25, a ridge 26, and a clad 27.
With the BR type laser device 20, it is difficult to fabricate the ridge in a reversed trapezoid form due to re-growth of the current blocking layer 25. Because the ridge is widened just above the active layer 22, electric current is not collected on one side but spread out, the unavoidable result of which is an unstable mode characteristic.